1. Field of the Invention
The present invention relates to a device having an element in which a luminous material is placed between electrodes (hereinafter referred to as light emitting element) (the device will hereafter be called a light emitting device). Specifically, the invention relates to a light emitting device having a light emitting element that employs as the luminous material an organic compound capable of providing EL (electro luminescence) (hereinafter referred to as EL element).
2. Description of the Related Art
In recent years, researches have been advanced on an EL element having a structure in which a thin film formed of an organic compound capable of providing EL (EL layer) is placed between an anode and a cathode, and light emitting devices utilizing the luminous characteristic of the EL element have been developed.
An EL layer usually has a laminate structure typical example of which is one proposed by Tang et al. of Eastman Kodak Company and composed of a hole transporting layer, a light emitting layer, and an electron transporting layer. This structure has so high a light emitting efficiency that it is employed in almost all of EL displays that are under development at present.
Other examples of the laminate structure of the EL layer include a structure in which a hole injection layer, a hole transporting layer, a light emitting layer, and an electron transporting layer are layered on an anode in this order, and a structure in which a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injection layer are layered on an anode in this order. The light emitting layer may be doped with a fluorescent pigment or the like.
In this specification all the layers that are placed between an anode and a cathode are collectively called an EL layer. Therefore the hole injection layer, the hole transporting layer, the light emitting layer, the electron transporting layer, and the electron injection layer mentioned above are all included in the EL layer.
When a given voltage is applied to the EL layer structured as above by a pair of electrodes, recombination of carriers takes place in the light emitting layer to emit light. A light emitting element composed of an anode, an EL layer, and a cathode is called herein an EL element.
In an EL element, degradation of its EL layer is accelerated when a driving voltage is high. Therefore an organic compound emitting Light by a triplet exciton (hereinafter referred to as triplet compound) is sometimes used instead of the usual luminous material, namely, a singlet compound (an organic compound that emits light by singlet exciton), because the triplet compound can emit light of high luminance with a low driving voltage.
The term singlet compound herein refers to a compound that emits light solely through singlet excitation and the term triplet compound herein refers to a compound that emits light through triplet excitation.
The luminance of light emitted from an EL element is controlled by the voltage applied to its EL layer. However, the luminance of emitted light in relation to the applied voltage varies between luminous materials used to form the light emitting layer in the EL layer. To elaborate, a luminous material that emits low luminance light requires application of high voltage if a higher luminance is aimed. Unfortunately, application of high voltage leads to degradation of the luminous material. Furthermore, if EL elements formed on the same substrate receive the same voltage but emit light of varying luminance, different voltages have to be applied in order to make the EL elements to emit light of the same luminance. This results in another problem of varying EL element lifetime.